The disposal of used tires for cars and trucks is a very large environmental problem for the U.S.A. and it is even more pronounced for some other densely populated countries all over the world. In the U.S. and Canada alone about 275,000,000 used tires are disposed of annually, usually in an open space. This is added to the estimated 2 to 3 billion tires already stockpiled in the U.S. The impact on the environment is enormous and represents health and fire hazards.
The used tires are composed of about 60% rubber, 40% steel and a small amount of plastic fiber. All these materials are reusable and recycling of used tires represents a very substantial new resource. The steel can be melted down, and the plastic fibers can be used several ways.
The use of rubber, the largest component of used tires, has received the most attention and it is fast expanding.
Rubber crumbs added to asphalt produce better and longer lasting roads. Other products such as floor tiles, rubber mats, tennis balls, trash cans, etc., are also made from crumb rubber. At this time there are several hundred developed applications for used rubber and the list is growing.
The development of rubber recycling technology started about two decades ago. However, the economical and environment-friendly recycling methods have been developed just recently. They can be classified as ambient grinding and cryogenic grinding methods. The final product of both methods is very similar, crumb rubber, by some called granules, ranging in size from 0.5 to 5 mm or ASTM classification 4-30 mesh. The larger sizes, 4 to 15 mesh, represent the most of production volume.
Some used rubber applications require much smaller sizes, 60-120 mesh, and the current price of this product is about $0.45/lb compared to about $0.15/lb for larger size crumb. Such price differences inspired development of methods to reprocess the crumbs to smaller size by secondary cryogenic freezing and grinding. The crumbs must be cooled to below -100.degree. F. to become hard and brittle, so that hammer-mill type equipment can be used to reduce the crumbs to a desired size.
Several methods, such as injecting liquid nitrogen in the mill, have been tried but the cost becomes prohibitive and it is not economically feasible. Several pounds of nitrogen are needed to freeze a pound of crumbs.